Safety belt systems were developed to secure the occupant of a vehicle against harmful movement that may result during a collision or a sudden stop. The systems reduce the likelihood and severity of injury in a traffic collision by stopping the vehicle occupant from hitting hard against interior elements of the vehicle or other passengers (the so-called second impact), by keeping occupants positioned correctly for maximum benefit from the airbag and by preventing occupants being ejected from the seat of the vehicle.
A traditional safety belt system comprises a safety belt for restraining the occupant in the seat, a belt winder for tightening or loosing the safety belt, and an electric motor for driving the belt winder. FIG. 6 illustrates a winding scheme of a traditional permanent magnet direct current (PMDC) motor used for driving the belt winder in a safety belt system. The rotor windings comprise a plurality of coils wound about teeth of the rotor core and electrically connected to segments of a commutator. For example, coil C1′ is wound about teeth T1 and T2 and connected to segments S1 and S2. Coil C2′ is wound about teeth T2 and T3 and connected to segments S2 and S3. The PMDC motor comprises two brushes to feed electricity to the rotor windings. The rotor windings form two parallel branches. When a coil such as coil C1′ is open, one of the branches will be open. The motor as well as the safety belt system will malfunction. It is dangerous to drive a car having a malfunctioning safety belt system.
Therefore, there is a desire for a safety belt system with improved reliability.